Bottom Fe Layer Research Articles

Design of multilayer planar film structures for near-perfect absorption in the visible to near-infrared.

In this work, a near-perfect broadband absorber, consisting of Fe, MgF2, Fe, TiO2 and MgF2 planar film, is proposed and investigated through simulations and experiments. The Fe material is first applied in the multilayer film structure, and it is proved to be more favorable for achieving broadband absorption. MgF2 and TiO2 are chosen as anti-reflection coatings to decrease unwanted reflections. The proposed absorber is optimized by employing a hybrid numerical method combining the transfer matrix method (TMM) and the genetic algorithm (GA). Under normal incidence conditions, the average absorption of the absorber is 97.6% in the range of 400 to 1400 nm. The finite difference time domain (FDTD) method and phase analysis reveal that the anti-reflection property and the Fabry-Perot resonance result in broadband absorption performance. Furthermore, when an additional Fe-MgF2 layer is inserted on the bottom Fe layer, an average absorption of 97.9% in the range of 400 to 2000 nm can be achieved. Our approach could be of vital significance for numerous applications involving solar energy.

Spin currents during ultrafast demagnetization of ferromagnetic bilayers

Ultrafast spin currents induced by femtosecond laser excitation of ferromagnetic metals have been found to contribute to sub-picosecond demagnetization, and to cause a transient enhancement of the magnetization of the bottom Fe layer in a Ni/Ru/Fe layered structure. We analyze the ultrafast magnetization dynamics in such layered structures by element- and femtosecond time-resolved x-ray magnetic circular dichroism, for different Ni and Fe layer thicknesses, Ru and Ta interlayers, and by varying the pump laser fluence. While we do not observe the transient enhancement of the magnetization in Ni/Ru/Fe discovered previously, we do find a reduced demagnetization of the Fe layer compared to a Ni/Ta/Fe layered structure. In the latter, the spin-scattering Ta layer suppresses spin currents from the Ni layer into Fe, consistent with previous results. Any spin current arriving in the lower Fe layer will counteract other, local demagnetization mechanisms such as phonon-mediated spin-flip scattering. We find by increasing the Ni and Fe layer thicknesses in Ni/Ru/Fe a decreasing effect of spin currents on the buried Fe layer, consistent with a mean free path of the laser-induced spin currents of just a few nm. Our results suggest that in order to utilize ultrafast spin currents in an efficient manner, the sample design has to be optimized with these considerations in mind, and further studies clarifying the role of interfaces in the employed layered structures are needed.

Modification of interlayer exchange coupling in Fe/V/Fe trilayers using hydrogen

Fe/V/Fe trilayers with constant-thickness Fe and step-like wedged V sublayers were prepared at room temperature using UHV magnetron sputtering. The bottom Fe layer grows onto oxidised Si(100) substrate and shows relatively high coercivity. The top Fe layer grows on vanadium spacer and shows considerably lower coercivity. The planar growth of the Fe and V sublayers was confirmed in-situ by X-ray photoelectron spectroscopy. Results show that the Fe sublayers are weakly exchange coupled for dV>1.4nm. Results on the coercivity studies as a function of the V interlayer thickness show near dV∼1.95nm (∼2.45nm) weak antiferromagnetic (ferromagnetic) coupling, respectively. The hydrogenation of the Fe/V/Fe trilayers leads to increase of the strength of the ferromagnetic interlayer exchange coupling.

Chirality switching and winding or unwinding of the antiferromagnetic NiO domain walls in Fe/NiO/Fe/CoO/Ag(001).

Fe/NiO/Fe/CoO/Ag(001) single crystalline films were grown epitaxially and investigated by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism. The bottom Fe layer magnetization is pinned through exchange coupling to the CoO layer and the top Fe layer magnetization can be rotated by an in-plane external magnetic field. We find that the NiO spins wind up to form a domain wall due to the perpendicular NiO/Fe interfacial coupling as the top layer Fe magnetization rotates from 0° to 90°, but switch wall chirality and unwind the wall as the Fe magnetization rotates from 90° to 180°. This observation shows that Mauri's 180° domain wall does not exist in perpendicularly coupled ferromagnetic-antiferromagnetic systems in the strong coupling regime.

Iron silicide formation at different layers of (Fe/Si)3 multilayered structures determined by conversion electron Mössbauer spectroscopy

The morphology and the quantitative composition of the Fe-Si interface layer forming at each Fe layer of a (Fe/Si)3 multilayer have been determined by means of conversion electron Mössbauer spectroscopy (CEMS) and high-resolution transmission electron microscopy (HRTEM). For the CEMS measurements, each layer was selected by depositing the Mössbauer active 57Fe isotope with 95% enrichment. Samples with Fe layers of nominal thickness dFe = 2.6 nm and Si spacers of dSi = 1.5 nm were prepared by thermal evaporation onto a GaAs(001) substrate with an intermediate Ag(001) buffer layer. HRTEM images showed that Si layers grow amorphous and the epitaxial growth of the Fe is good only for the first deposited layer. The CEMS spectra show that at all Fe/Si and Si/Fe interfaces a paramagnetic c-Fe1−xSi phase is formed, which contains 16% of the nominal Fe deposited in the Fe layer. The bottom Fe layer, which is in contact with the Ag buffer, also contains α-Fe and an Fe1−xSix alloy that cannot be attributed to a single phase. In contrast, the other two layers only comprise an Fe1−xSix alloy with a Si concentration of ≃0.15, but no α-Fe.

Magneto-optical properties of Fe/Cr/Fe/MgO/Fe structures epitaxially grown on GaAs(001)

Fe/Cr/Fe trilayers were epitaxially grown on atomically flat GaAs(001). For the thickness of Cr spacer layer corresponding to antiferromagnetic coupling, “reversed” minor hysteresis loops were measured with longitudinal magneto-optical Kerr effect (MOKE), i.e., a negative “magnetization” signal was detected when the thicker bottom Fe layer was saturated along the applied field. This behavior is interpreted by depth variations of the MOKE sensitivity. Magnetization reversal shows that both antiferromagnetic switching and spin–flop transition fields depend on the ratio of both Fe film thicknesses. The shape of the MOKE loops becomes more complex with further deposition of MgO and Fe layers on the top of the Fe/Cr/F/GaAs(001) stack. Superconducting quantum interference device measurements confirm the interpretation of the MOKE loops and demonstrate homogeneity and sharpness of the interfaces in the structures.

Magnetic profiles and coupling in Fe/Cr(110) superlattices

In epitaxial Fe/Cr superlattices the coupling between the Fe layers oscillates between antiferromagnetic (AFM) and ferromagnetic as a function of the Cr layer thickness tCr. The period of the oscillation is the same for superlattices grown with (211) and (100) orientations. We measured the coupling of Fe/Cr(110) superlattices consisting of three identical Fe layers. The magnetization curves were characterized by two to four levels, M/Ms=±1 or ± $\frac\Box1\Box\Box3\Box$ . Polarized neutron reflectometry identified the direction of the magnetization of each Fe layer and showed that the levels M/Ms=± $\frac\Box1\Box\Box3\Box$ were not always due to AFM alignment of the central layer, but rather it was found that, in spite of the structural similarity, the bottom Fe layer had a different coupling strength from the top Fe layer. The magnetic coupling must be sensitive to small structural differences at the interface. Furthermore, the measurements indicate an unexpected periodicity for Fe/Cr(110) superlattices.